In accordance with reduction of sizes and increase of speeds of computers in recent years, magnetic disk devices used as external storage devices have been required to reduce the sizes and increase the capacities and operation speeds. In view of this, thin film magnetic heads capable of high density recording with a low inductance have been earnestly developed for use as the magnetic heads for magnetic disk devices.
Such a thin film magnetic head includes a substrate 1 as shown in FIG. 20, and also includes an insulating film 2, a lower magnetic core 3, a gap spacer 7, a conductive coil layer 6 covered with a coil layer insulating film 4, an upper magnetic core 31 and a protection layer 21 which are layered in this order on the substrate 1.
The substrate 1 is made of ceramics such as AL.sub.2 O.sub.2 --TiC. The insulating film 2 and gap spacer 7 are made of insulating substance such as SiO.sub.2 or AL.sub.2 O.sub.3. The magnetic cores 3 and 31 are made of magnetic material such as Ni--Fe, Fe--Al--Si or Co--Zr--Nb.
The aforementioned layers or films forming the thin film magnetic head are respectively formed by photolithography, electroplating, vapor deposition or other known thin film forming technique.
In a process of forming the magnetic film, vapor deposition or sputtering is effected in a magnetic field to induce a uniaxial anisotropy in a direction of a track width so as to achieve a high magnetic permeability in a high frequency area. It has been proposed in Unexamined Japanses Patent Publication HEI.3-181010 that magnetic anisotropy 1s applied to a surface of a magnetic film by thermal processing or others in a magnetic field after forming the film.
The thin film magnetic head having the uniaxial anisotropy induced in the magnetic core performs signal recording and signal reproduction by rotation of a magnetization direction, so that it can reduce noises compared with the case where domain walls move (see IBM J. RES. DEVELOP, Vol. 34, No. 6, November 1990, pp 884-891).
When forming the upper magnetic core 31 shown in FIG. 20, a magnetic film having the unidirectional anisotropy is formed on the whole surface of the insulating film 4, and then the magnetic film is patterned by ion beam etching or the like as shown in FIG. 21. In this patterning step, it is inevitable that crystals are disarranged at the edge of the pattern, resulting in deterioration of characteristics.
As a result, fluctuation generates in the unidirectional anisotropy induced immediately after the film formation, and thereby triangular reflux domains 91 generate, or the anisotropy applied in the film forming process is cancelled, resulting in a transient domain structure.
The thin film magnetic head including the magnetic core having the aforementioned domain structure may cause so-called wiggle noises or popcorn noises due to movement of the domain walls during reproduction of signals on the magnetic recording medium (see Summaries of 15th Scientific Lecture of Japan Application Magnetism Society, p. 19, and Summaries of 14th Scientific Lecture of Japan Application Magnetism Society, p. 163).
As a result, distortion may generate in a reproduced waveform, which frequently causes an error in reproduced information.
Further, substance of the magnetic core is restricted to material in which unidirectional anisotropy can be induced in the track width direction, i.e., material such as Ni--Fe in which magnetic anisotropy is not induced to a large extent. Material having a large saturation flux density cannot be used as the above substance. Therefore, the recording capability of the thin film head is restricted.